Consistency imposes a rule that only valid data can be written to a database. When an update is executed, the database state will change only if all the database consistency rules are obeyed, otherwise the transaction will be rolled back and the database restored to its consistent state. On the other hand, if all the updates are consistent, the database will be taken to a new consistent state.
Consistency rules can include: data type correctness, relation correctness, uniqueness of data, etc.